Microphones insensitive to external noise and picking up only a speaker's voice include a bone conduction microphone and a throat microphone. A typical bone conduction microphone for picking up bone vibration is an acceleration pickup microphone which uses a piezoelectric element supported by a supporting portion, as shown in FIG. 8.
This type of microphone is in common use since it is highly sensitive while resistant to external noise, but it has drawbacks that it is sensitive to sliding noise and its frequency response is not flat (a large resonance peak for its element occurs).
Also, a dynamic microphone is known, as shown in FIG. 9. The dynamic microphone has advantages of being resistant to sliding noise and easy to use because the frequency response thereof has no large peak. However, the dynamic microphone has drawbacks that it has low sensitivity and is large structurally. This type of dynamic microphone is mainly used as a throat microphone for picking up vocal fold vibration, having a thicker diaphragm to decrease sensitivity to air-conduction sound.
Further, a type of vibration pickup microphone is known, in which bone vibration or vocal fold vibration picked up by its diaphragm are converted to air vibration in an air chamber at the front of a microphone unit such as an electret microphone and then the air vibration is picked up (see FIG. 10).
This type of microphone has high microphone sensitivity and resistance to sliding noise. This is because the vibration is picked up only by its diaphragm, and any portions other than the diaphragm have extremely low sensitivity to vibration. Accordingly, this type of microphone can be resistant to sliding noise.
In this way, though this type of microphone has advantages of having high microphone sensitivity and resistance to sliding noise, it has also a drawback that its anti-noise characteristic is somewhat degraded when used in high ambient noise environments. That is, when the ambient noise level exceeds 110 dB SPL, this type of microphone is more likely to be affected by noises in bands higher than voice bands due to the characteristic of condenser microphones that its frequency range is wider than that of voice band.    Patent Document 1: Japanese Patent Laid-Open No. 2006-20247    Patent Document 2: Japanese Patent Laid-Open No. 2004-229147    Patent Document 3: Japanese Patent Laid-Open No. 2001-292489